﻿84 



MR. G. W. TYRRELL ON THE 



[vol. lxxii, 



7. The Picrite-Teschentte Sill of Lugar (Ayrshire). By 

 George Walter Tyrrell, A.K.C.Sc, F.G.S., Lecturer in 

 Mineralogy and Petrology in the University of Glasgow. 

 (Read April 5th, 1916.) 



[Plates X & XI.] 



Contents. 



I. Introduction 84 



II. Field Eelations 85 



(1) The G-lenmuir Section. 



(2) The Bellow Section. 



(3) Other Exposures. 



(4) Summary. 



III. Petrography 95 



(1) The Contact-Bocks. 



(2) Teschenite. ; 



(3) Theralite. 



(4) Lugarite. 



(5) Picrite and Peridotite. 



IV. Petrology 114 



(1) Mineralogical Variations. 



(2) Chemical Variations. 



(3) Average Magma of the Lugar Sill. 



(4) Composition, Identity, and Banding of the 



Contact-Rocks. 



(5) Asymmetry. 



(6) Density-Stratification. 



(7) Variations in Texture. 



(8) Segregation-Veins. 



(9) Mode of Intrusion and Differentiation. 



(10) Sinking of Crystals in the Central Ultrabasic 



Stratum. 



(11) Comparison of the Lugar Sill with the other 



Picrite- Teschenite Sills of Scotland. 



I. Introduction- 



The association of teschenite and ultrahasic rocks (picrite and 

 peridotite) in a single rock-body has now been established for 

 several occurrences in the knvlands of Scotland. The Barnton 

 occurrence, near Edinburgh, has been described by Sir Archibald 

 Geikie, 1 by Mr. J. Henderson & Mr. J. G. Goodchild, 2 and by 

 Mr. H. W. Monckton. 3 At Blackburn, near Bathgate, occurs a 

 picrite, which has been described by the first-named writer as a 

 lava, 4 but has recently been shown to be intrusive and associated 

 with teschenite by the officers of the Geological Survey of Scotland. 5 



1 ' Ancient Volcanoes of Great Britain ' vol. i (1897) pp. 449-50. 



2 Trans. Geol. Soc. Edin. vol. vi (1893) pp. 297-300 & 301-302. 



3 Q. J. G. S. vol. 1 (1894) p. 39. 



4 ' Ancient Volcanoes of Great Britain' vol. i (1897) p. 419. 



5 ' Summary of Progress for 1904' Mem. Geol. Surv. 1905,^. 118-19. 



